1. Field of the Invention
The present invention relates to a transmitter used for a communication apparatus such as a mobile phone and a wireless LAN, and particularly to a transmitter for outputting a transmitted signal with low distortion and at high efficiency even in a wideband modulation scheme and to a communication apparatus using the same.
2. Description of the Background Art
Conventionally, as a radio-frequency power amplifier for amplifying a modulated signal including an envelope fluctuation component, a class-A or class-AB linear amplifier is used to linearly amplify the envelope fluctuation component. Such a linear amplifier has high linearity, but, on the other hand, constantly consumes the electrical power resulting from a DC bias component, and therefore has lower power efficiency than that of, for example, a class-C or class-E non-linear amplifier. Therefore, when this radio-frequency power amplifier is applied to a battery-powered mobile communication apparatus, the operating time decreases due to a large amount of power consumption of the radio-frequency power amplifier. When the radio-frequency power amplifier is applied to a base station device of a wireless system where a plurality of high-power transmitters are provided, the device increases in size and the heating value increases.
In response, as a transmitter that operates at high efficiency, conventionally, a transmitter applying polar modulation is proposed. FIG. 20 is a block diagram showing a conventional transmitter 500 applying polar modulation. Referring to FIG. 20, the conventional transmitter 500 includes a polar coordinate signal generation section 501, an angle modulation section 502, a regulator 504, and an amplitude modulation section 505.
The polar coordinate signal generation section 501 generates an amplitude signal and a phase signal. The amplitude signal is inputted to the regulator 504 and the phase signal is inputted to the angle modulation section 502. To the regulator 504, a direct-current voltage is supplied. The regulator 504 supplies to the amplitude modulation section 505 a voltage corresponding to the inputted amplitude signal. The angle modulation section 502 generates an angle-modulated signal by angle-modulating the inputted phase signal and outputs the angle-modulated signal to the amplitude modulation section 505. The amplitude modulation section 505 amplitude-modulates, by the voltage supplied from the regulator 504, the angle-modulated signal outputted from the angle modulation section 502 and outputs the resulting signal as a modulated signal. This modulated signal is outputted as a transmitted signal.
Additionally, in the transmitter applying polar modulation, distortion may occur in a transmitted signal due to nonlinearities of the regulator 504 and the amplitude modulation section 505. In response, conventionally, a method of compensating for the nonlinearities of the regulator 504 and the amplitude modulation section 505 in the transmitter applying polar modulation by a compensation filter and thus decreasing the distortion of the transmitted signal is disclosed (see International Publication no. WO/2006/118317, for example).
FIG. 21 shows a conventional transmitter 600 using this method. The conventional transmitter 600 has a structure where the conventional transmitter 500 further includes a compensation filter 601. The compensation filter 601 has the inverse characteristics of the transfer characteristics from the input of the regulator 504 to the output of the amplitude modulation section 505 and compensates for the frequency characteristics of the regulator 504 and the amplitude modulation section 505 by performing waveform shaping on the amplitude signal generated by the polar coordinate signal generation section 501. This makes it possible to suppress the distortion caused by the insufficiency of the frequency bandwidths of the regulator 504 and the amplitude modulation section 505 and to realize a transmitter that operates with low distortion and at high efficiency even in a wideband modulation scheme.
The structure of the regulator 504 and the amplitude modulation section 505 described above can be represented by, as shown in FIG. 22, an equivalent circuit where a low-pass filter (LPF) 512 is provided between an amplification section 511 and a multiplication section 513. It is found that the cutoff frequency may change due to the LPF 512, depending on the magnitude of the amplitude signal outputted from the amplification section 511. Therefore, there remains the problem that, in the conventional transmitter 600 using the compensation filter 601 having a fixed value, it is difficult to compensate for the frequency bandwidths of the regulator 504 and the amplitude modulation section 505 and thus it is impossible to sufficiently suppress the distortion of the transmitted signal.